166 GENETICS 



6. MODIFYING GENES AND SELECTION 



The recognition of modifying genes has furnished 

 an explanation for the apparent effectiveness of selec- 

 tion within a pure line without assuming germinal 

 inconstancy. 



The gene itself, like that producing the hooded pat- 

 tern of Castle's rats, is constant but it is accompanied 

 by a halo of modifying genes likewise constant which 

 have no somatic expression except when the original 

 factor for hooded pattern is present. These modifying 

 genes are simply potential increasers or diminishers of 

 the hooded-pattern gene. In the absence of the pattern 

 gene there is nothing to increase or diminish and con- 

 sequently there is no way to demonstrate the modifying 

 factors. They are not imaginary things, however, for 

 their separate existence and transmissibility have been 

 demonstrated from many sides. What selection within 

 the progeny of the homozygous cross of hooded rats 

 or bristly flies accomplishes is simply the elimination 

 or addition of either plus or minus modifying genes, 

 according as the attempt is being made to increase 

 or decrease the hooded pattern of pigmentation or the 

 number of bristles. 



If this explanation stands the test of further investi- 

 gation then we are still dealing in heredity with con- 

 stant dependable units, such as the chemist finds in his 

 elements, and it may be said that all genetic roads 

 lead to the Rome of gene-constancy. 



However, it is well to remember that Darwin did not 

 revolutionize the concept of evolution until he broke 



